Any time you have sex w/her there is a chance she can get pregnant! Is she on birth control?
Maybe its just a little late.....I didn't have a pieriod one month, and I wasn't preg!
Anything can throw off a menstural cycle, has she lost weight? Is she under stress? There are many other things that can throw it off.
But she could be preg! Its not the worst thing in the world to come clean with your parents, they might actually be able to give you some good advice.
If she is preg, its both of your guys choice what to do with the baby, make sure shes totally comfortable with the abortion, and she should get counsuling after it!!! An abortion can be extremly emotional!